Now turn back to the 2500 router in Building A of the San Francisco campus in Figure 5-6. The router is now happily a member of the OSPF network. Now follow the process of hearing an update to the network in the form of an LSA.
As soon as a router realizes that there has been a change in the network topology, the router is responsible for informing the rest of the routers in the area. Typically, it will identify a change in the state of one of its links for one of the following reasons:
• The router loses the physical or data link layer connectivity on a connected network. It will propagate an LSU and send it to the DR on a multiaccess network or the adjacent router in a point-to-point network. From there, it is flooded to the network.
• The router fails to hear either an OSPF Hello protocol or a data link Hello protocol. It will propagate an LSU and send it to the DR on a multiaccess network or the adjacent router in a point-to-point network. From there, it is flooded to the network.
• It receives an LSA update from an adjacent neighbor, informing it of the change in the network topology. The LSU is acknowledged and flooded out the other OSPF interfaces.
In any of these instances, the router will generate an LSA and flood it to all its neighbors.
This discussion now turns to the process initiated when a router receives such an update. For this purpose, return to the 2500 connected to its designated router, the 7200.
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